1. Field of the Invention
The present invention relates to a filter circuit, for example, a band limiting filter circuit provided in a posterior stage of a power amplifier in a transmission unit of a radio communication apparatus.
2. Related Art
Conventionally, a filter circuit is constructed by cascade-connecting resonators (resonance circuits) whose conductor part is made of, for example, superconductor. A superconductor has a limit value in a current per unit area that can flow in a superconducting state. An equivalent circuit of a resonator is made up of an inductor and a capacitor and is also provided with a resistor when an effect of loss is considered. A resonance frequency f0 of a resonator when there is no resistor is given by the following expression. “L” and “C” denote inductance and capacitance of the resonator respectively.f0=(L×C)1/2 
In this filter circuit, a pass frequency range and an amount of attenuation of a filtering region can be determined by appropriately determining an inter-resonator coupling coefficient which indicates the amount of coupling between resonators and a value of external Q which indicates an amount of excitation for resonators on the input side and the output side.
In such a filter circuit made up of cascade-connected resonators, a current flows through each resonator, that is, a current of all frequency components flows through each resonator, and therefore power handling capability of each resonator needs to be increased. This results in a problem of increasing the size of the circuit. The specification of U.S. Pat. No. 6,633,208 describes that a highest current passes through a first resonator in a cascade connection type filter circuit, and a multi-wavelength structure is adopted for the first resonator (i.e. line length is set to half wavelength×n (n is an integer equal to or greater than 2)) to disperse the current in the resonator.
On the other hand, as another filter circuit, there is a parallel connection type filter circuit made up of resonators connected in parallel whose conductor part is made of superconductor (see, for example, JP-A 2001-345601 (Kokai) and JP-A 2004-96399 (Kokai)). This parallel connection type filter circuit combines signals which pass through resonators having neighboring resonance frequency so as to have phases opposite to each other and thereby realizes a filter characteristic. This filter circuit distributes input power to the respective resonators, and can thereby increase the power handling capability as a whole, yet reduce power handling capability of each resonator compared to a cascade connection type filter circuit and thereby also reduce the circuit scale. However, there is a demand for a further reduction in the circuit scale.